1. Field of the Invention
The present invention relates to an exposure apparatus, an exposure method, and a device fabrication method.
2. Description of the Related Art
A projection exposure apparatus has conventionally been employed to fabricate, for example, a micropatterned semiconductor device such as a semiconductor memory or logic circuit by using photolithography. The projection exposure apparatus transfers a circuit pattern formed on a reticle (mask) onto, for example, a wafer via a projection optical system.
A minimum dimension (resolution) that a projection exposure apparatus can transfer is proportional to the wavelength of exposure light and is inversely proportional to the numerical aperture (NA) of a projection optical system. The shorter the wavelength and the higher the NA, the better the resolution. Along with the recent demand for micropatterning semiconductor devices, the wavelength of exposure light is shortening and the NA of a projection optical system is increasing. For example, to shorten the wavelength of exposure light, an ArF excimer laser (wavelength: about 193 nm) or an F2 laser (wavelength: about 157 nm) has become used as an exposure light source nowadays instead of a conventional KrF excimer laser (wavelength: about 248 nm). To increase the NA of a projection optical system, an optical system with an NA greater than 0.90 is under development.
Under the circumstances, immersion exposure is receiving a great deal of attention as a technique for further improving the resolution while using a light source such as an ArF excimer laser or F2 laser. The immersion exposure further increases the NA of a projection optical system by using a liquid as a medium which fills the space under the projection optical system on the wafer side (image plane side). The NA of the projection optical system is NA=n·sin θ where n is the refractive index of the medium. It is therefore possible to increase the NA to n by filling at least part of the space between the projection optical system and the wafer with a medium (liquid) having a refractive index (n>1) higher than that of air. In other words, the immersion exposure improves the resolution by increasing the NA of the projection optical system seen from the wafer side (by 1 or more).
The exposure apparatus also has a sensor for measuring an optical characteristic and performs various types of mechanical adjustment and optical adjustment based on the output from the sensor, thereby optimizing the wafer exposure operation (exposure condition). For example, the exposure apparatus has an illuminance nonuniformity sensor or irradiation dose sensor on a wafer stage which supports a wafer. The illuminance nonuniformity sensor measures illuminance nonuniformity (light amount distribution) of exposure light having passed through the projection optical system. The irradiation dose sensor measures the irradiation dose (light amount) of exposure light having passed through the projection optical system.
These sensors receive light via a transmission part (measurement mark) formed on the image plane side of the projection optical system. As the numerical aperture of the projection optical system increases (the incident angle of exposure light with respect to the wafer increases) by immersion exposure, the angle of divergence of the light which emerges from the transmission part also increases. For this reason, the above-described sensors often cannot receive full-aperture light. However, the above-described sensors can receive full-aperture light by curving the exit side of a measurement substrate on which a transmission part is formed (drawn) or by arranging a planoconvex lens with a curvature to be in contact with or adjacent to a measurement substrate (i.e., by decreasing the angle of divergence of light). These techniques are proposed in Japanese Patent Laid-Open Nos. 2005-175034, 2005-268744, and 2005-129914.
Unfortunately, when the exit side of a measurement substrate is curved or a planoconvex lens with a curvature is arranged in contact with or adjacent to a measurement substrate, light at an off-axis position along the optical axis (the center of curvature) is distorted due to the curvature, although that at an on-axis position is free from any such influence. If the measurement substrate suffers a manufacturing error such as a drawing error generated upon drawing a transmission part or a positioning error of a planoconvex lens to be arranged in contact with or adjacent to the measurement substrate, incident light is distorted irrespective of whether it is on- or off-axis.
To measure a plurality of optical characteristics by one sensor, it is necessary to form a plurality of transmission parts corresponding to the respective characteristics on the measurement substrate. In this case, no problem is posed as long as the measurement substrate is a parallel plate. However, as described above, when the exit side of the measurement substrate is curved or a planoconvex lens with a curvature is arranged in contact with or adjacent to the measurement substrate, incident light is distorted depending on the positions (coordinate positions) of the transmission parts on the substrate.
Upon receiving light distorted due to such factors, the sensors output optical characteristic measurement results with errors (measurement errors), resulting in significant decreases in measurement precision. If a plurality of transmission parts is formed on a measurement substrate, regions where these transmission parts can be formed are limited under the influence of light distortion.